1. Field of the Invention
This invention relates to a process for producing sorbic acid. More particularly, it relates to a process for sorbic acid commercially and advantageously, from a polyester obtained from the reaction of crotonaldehyde with ketene (hereinafter referred to merely as polyester).
2. Description of the Related Art
Sorbic acid or its salts either have superior fungiresistance; hence they have been preferably used as a food preservative.
The process for producing sorbic acid includes those of reacting crotonaldehyde with ketene to obtain a polyester via .beta.-lactone as an intermediate, followed by subjecting the polyester to acidolysis, thermal decomposition or alkaline decomposition to produce sorbic acid, and among these processes, acidolysis process has generally been often carried out.
As the acidolysis process, a mineral acidolysis process using a mineral acid such as hydrochloric acid or sulfuric acid as a catalyst, a decomposition process with an ion exchange resin using as a catalyst, an acidic giant reticulated ion exchange resin (MR) having pores of at least 200 .ANG. on an average and a water content of 20% by weight or less (Japanese patent publication No. Sho 57-47,655), or the like processes are known.
According to the mineral acidolysis process, there are advantages that the polyester is not only decomposed into sorbic acid, but also a byproduced cis-isomer is isomerized into sorbic acid, but a problem is raised that a black-brown byproduct is formed simultaneously with the polyester decomposition, whereby crystals of sorbic acid are contaminated to make their purification difficult. Further, an additional problem is raised that catalyst-mineral acid water is contaminated to make its circulation use difficult; hence the mineral acidolysis process cannot always be regarded as a commercially advantageous production process.
On the other hand, the decomposition process with an ion exchange resin is superior to the mineral acidolysis process in that the quantity of the black-color byproduct formed is very small, but the present inventors found as a result of the tracing test of the former process, a problem that the ion exchange resin loses its catalytic activity within such an unexpectedly short time that the catalyst cannot be practically used.
In order to solve the above problem, we attempted to restore the catalytic acitivity by adding a mineral acid, as carried out in the prior art, but the restoring effect was small and to the contrary, various new problems occurred such as a problem of formation of a balck-brown byproduct due to the mineral acid, a problem of corrosion of equipment, and a problem that since the added mineral acid mixes in crystals of sorbic acid, it is necessary to remove the acid. The decomposition process with an ion exchange resin has a problem in that it is difficult to control the water content of the ion exchange resin used.
Namely, in general, ion exchange resins have been commercially availed in a hydrous state of a water content in the vicinity of 50%, and when the water contained in the ion exchange resins is removed by means of azeotropic distillation, etc. as much as possible, in order to enhance the catalytic acitivity, then the catalytic activity is entirely lost, while when the water content is increased to the contrary, the catalytic activity is lost after a certain time. Thus, the subject to be solved by the invention is to provide a more advanced process for producing sorbic acid, having overcome the above-mentioned problems.
The present inventors have examined the reduction in the catalytic activity as one of the above problems in the case of the decomposition process with an ion exchange resin, and as a result have found that the reduction is deeply related to impurities contained in the polyester, particularly to zinc, that is, when the polyester is prepared from crotonaldehyde and ketene, an ion exchange occurs between a zinc salt and the exchange group of the exchange resin to reduce the acid concentration of the ion exchange resin and thereby lose the catalytic function of the resin and as a result, reduce the percentage of decomposition of the polyester into sorbic acid.
In order to recover the lost catalytic function of the ion exchange resin, it is necessary to wash the resin with about 5 to 10% hydrochloric acid, sulfuric acid or the like to dissolve away zinc, but as a result, since the ion exchange resin contains a large quantity of water, a cumbersome matter occurs that the resin should be dehydrated up to an adequate water content.
Thus, the present inventors have made extensive research in order to find a process of not damaging the catalytic life of the ion exchange resin, and as a result have found that when the polyester is first dissolved in a solvent, followed by washing the resulting solution with a mineral acid water and, if the zinc salt is water-soluble, washing it with water in advance, the object can be easily achieved.
Next, the present inventors have made extensive research into the catalytic acitivity of the ion exchange resin, and as a result, have found that when the ion exchange resin is treated with, preferably washed with a lower alkyl alcohol to replace a portion of water by the lower alkyl alcohol, the object can be easily achieved without the above difficult control of the water content.
Namely, the ion exchange resin retaining a lower alkyl alcohol in the pores of the resin has a superior affinity to the solution of the polyester dissolved in the solvent, as compared with the ion exchange resin retaining water in the pores of the resin; hence the former resin easily contacts with the solution of the polyester dissolved in the solvent so that decomposition of the polyester raises no obstacle.
This washing process with a lower alkyl alcohol removes alcohol-soluble impurities present in the ion exchange resin, and at the same time the process is applicable to a process for regenerating the ion exchange resin in the case where impurities such as ketene polymer, crotonaldehyde polymer, etc. remaining in the polyester adhere to the ion exchange resin during decomposition of the polyester to reduce the catalytic activity. The present inventors have solved these problems and have completed the present invention.